Preformansce profile

Data on cooling capacity, efficiency and general performance provided by the manufacturer, or even independent test facilities, cannot be said to apply to the UK. However, these are often the data used when buying and installing small-scale air conditioning systems in the domestic market.

The aim of BSRIA's research was to measure and quantify the energy performance of small-scale air conditioning systems by means of a laboratory-based test regime. The research team used typical UK climate and operating conditions to measure the performance of the equipment. Data pertaining to the UK weather profile can then be used to determine the best product for the situation.

Test conditions

The test conditions for typical UK summer heat loads were based on the CIBSE Guide and data from the Meteorological Office in Bracknell. The test methods are generally based on the methods given in the various standards. The results are given in table 2.

All the types of air conditioning system tested, including the dehumidifiers, performed more efficiently at the test condition for the UK weather profile (T3) than they did at the design specification (T1). This outcome is at odds with conventional wisdom that machines running at part load do so inefficiently.

Generally, manufacturers state the performance for their products at the T1 condition. The average measured performance of the units tested at T1 was 82% of the data specified by the manufacturer. The exceptions were the portables, which had cooling capacities significantly less than the specified data ie 29%.

Portables produce more cooling than stated here, but some of the cooling produced is used to counter the heat that remains in the room from the unit's exhaust, and this is not included in the values. However, this clearly shows one of the main inefficiencies of portable units.

The total cooling provided by all the units at the T3 condition was, on average, 91% of the total cooling provided at the T1 design condition. This means that while the manufacturer will claim an output at a notional condition (T1), the unit, when running in more typical UK conditions (T3), will only produce 75% of that claimed output.

The superior performance of the split units as measured may be due to the condenser and evaporator being physically separated. However, the distance between the two units, and in particular vertical separation, can have a very dramatic effect on performance. Generally, the heat given off by the split unit condenser (outside the building) does not affect the cooling at the evaporator.

The portables have the condenser and evaporator in the same casing, so that the heat given off by the condenser can affect the cooling at the evaporator.

Window units are a compromise between splits and portables, with the evaporator and condenser in the same casing, but with the evaporator indoors and the condenser outdoors. The effect of the heat from the condenser on the evaporator is therefore minimised, as the heat escapes to the outside.

The dehumidification systems under test operated by air passing over a low temperature heat transfer surface (the evaporator), so that heat was transferred from the air to the refrigerant, and moisture extracted as the air was cooled to a temperature below its dew point. The air was then reheated by passing it over the refrigerant condenser coil.

Overall performance

At typical UK conditions (T3), the air conditioner units produce less cooling than at their design (T1) conditions, but with a marginal increase in the coefficient of performance (cop).

The total cooling provided by the units at the T3 condition was, on average, 91% of the total cooling provided at the T1 condition, that is the cooling capacity of the units was on average 75% of the manufacturer's specification

Portable units were somewhat less effective than split or window units at the T1 condition and had a cooling capacity of only 29% of the manufacturer's specification. At the UK (T3) conditions for example the situation worsens, a 2 kW portable unit only delivered 0·52 kW of useful cooling.

Inverter-driven units gave lower average total cooling at the T3 condition, typically 65% of the T1 specification compared to the non-inverter units. However, their power consumption was also significantly less, which resulted in overall higher efficiency.

The single split inverter-driven units tested were characterised by maintaining a constant air-off temperature, regardless of load; as air-on conditions fall, so the cooling delivery falls. As long as the unit is sized for the maximum cooling likely, then the unit will provide cooling up to that level, and the efficiency will be as good as the manufacturer's specification or better.

The temperature of the air leaving the air conditioner units was on average 12°C below the room temperature, which may cause draughts to be perceived. Temperature differences as high as 20 K also occurred, which would definitely cause thermal discomfort. The inverter units generally had higher air-off temperatures and better cop when operating at part load.

Further reading

ISO 5151:1994 Non-ducted air conditioners and heat pumps — testing and rating for performance.

BS EN 814:1997 (Parts 1, 2 and 3) British and European standard for air conditioners.

BS EN 810:1997 Dehumidifiers with electrically-driven compressors.

Hawthorne C, 'Design issues for a/c split systems', Building Services Journal, 3/99.

CIBSE Guides A, B and C, CIBSE, ISBN 0 900953 28 4.

BSRIA Guide to the efficient use of small air conditioning, BSRIA, 1999.

This work was part-funded by the DETR under the Partners in Innovation scheme. The work was also sponsored by Air Trembath, Carrier Holdings, Hitachi Europe and S&P Coil Products.